Update the README.md to refer to build-wasm.sh and point to the developer docs

[gregtatum]

When I was onboarding, it was hard to find the docs, and the Wasm building documentation was out of date. I made the Wasm abbreviation consistent on this page, as the official spelling is Wasm per https://webassembly.org/

This should hopefully help the next person along get everything built and working.

[kpu]

If you want Wasm in documentation, there's also CMakeLists.txt and shell script comments and help strings...

[gregtatum]

@kpu Done.